1. Field of the Invention
One or more exemplary embodiments relate to a magnetic jack type control element drive mechanism for precision position control of a control element assembly, and more particularly, to a control element drive mechanism which is applied to a 4-coil magnetic jack type control element drive mechanism to increase resolution of position control of a motor assembly.
The present invention is derived from research conducted as part of the Nuclear Power Core Technology Development Program by the Ministry of Trade, Industry & Energy [Project Serial Number: 20131510101680, Title of Research Project: Development of Top-Mounted ICI System and In-Vessel Control Element Drive Mechanism for Severe Accident Mitigation Design.
2. Description of the Related Art
A control element drive mechanism is provided to control the power of a nuclear reactor and is classified as, for example, a magnetic-jack type control element drive mechanism, a ball-screw type control element drive mechanism, and a hydraulic type control element drive mechanism. The present invention relates to a magnetic jack type control element drive mechanism.
FIG. 1 is a conceptual diagram of a control element drive mechanism used for installation thereof, and FIG. 2 is a schematic cross-sectional view of a conventional control element drive mechanism. FIG. 3 is a magnified cross-sectional view illustrating the main portion of FIG. 2, and FIG. 4 shows a sequence of a control element being withdrawn by a conventional control element drive mechanism.
As shown in FIG. 1, a nuclear fuel assembly 2 and a control element 3 are placed in a nuclear reactor 1. The control element 3 controls the fission of the nuclear fuel by adjusting the number of neutrons absorbed by a nuclear fuel. The control element 3 is connected to a control element drive shaft 6. The control element 3 is vertically driven up and down by a control element drive mechanism 5. For the installation of the control element drive mechanism 5, a nozzle 4 is placed on an upper portion of the nuclear reactor 1.
As shown in FIG. 2, the control element drive mechanism 5, which may be a 4-coil type control element drive mechanism, includes an upper motor assembly 10, a lower motor assembly 20, and a control element drive coil.
The upper motor assembly 10 includes an upper latch 13, an upper stationary magnet 14, an upper lift magnet 15, and an upper latch magnet 16. Meanwhile, the lower motor assembly 20 includes a lower latch 23, a lower stationary magnet 24, a lower lift magnet 25, and a lower latch magnet 26.
The control element drive coil includes an upper lifting (UL) coil 11, an upper gripper (UG) coil 12, a lower lifting (LL) coil 21, and a lower gripper (LG) coil 22. The control element drive mechanism 5 controls the vertical movement of the control element 3 by controlling the magnetic force generated by the four coils.
In detail, a 4-coil magnetic jack type control element drive mechanism operates a motor assembly in a double-step manner. A first step occurs at the upper motor assembly 10, and a second step occurs at the lower motor assembly 20. The first step and the second step constitute a pitch.
Operation of the motor assembly in the double-step manner will be explained in detail by referring to FIG. 4, as follows. For the convenience of explanation, a sequence of withdrawing a control element is provided herein. A process of inserting the control element will be carried out in the reverse order of the withdrawing sequence.
The first step is completed by the operation of the upper motor assembly 10. In detail, when current is provided to the UG coil 12, an upper latch 13 engages with teeth 7 of the control element drive shaft 6, whereafter current is provided to the UL coil 11 and the upper lift magnet 14 ascends to drive up the control element drive shaft 6. When current is applied to the LG coil 22, the lower latch 23 engages with the teeth 7 of the control element drive shaft 6, whereafter the current supply to the UL coil 11 and the UG coil 12 is blocked to make the control element drive shaft 6 remain elevated.
Meanwhile, the second step is completed by the operation of the lower motor assembly 20. In detail, following a last phase of the first step, current is provided to the LL coil 21 and a lower lift magnet 25 is driven up, and current is provided to the UG coil 12 and the upper latch 13 engages with the teeth 7 of the control element drive shaft 6, whereafter the current supply to the LL coil 21 and LG coil 22 is blocked. As a result, the upper latch 13 is engaged with the teeth 7 of the control element drive shaft 6 to make the control element drive shaft 6 remain elevated.
As explained before, the conventional control element drive mechanism completes one pitch of ascending or descending the control element drive shaft 6 only when the first and second steps are all completed.
As shown in FIG. 3, a lift gap (d1) of the upper motor assembly 10 and a lift gap (d2) of the lower motor assembly 20 are 7/16 of an inch and ⅜ of an inch, respectively. When the upper motor assembly 10 and the lower motor assembly 20 operate a lifting operation, a space margin between the upper latch 13 and the teeth 7 of the control element drive shaft 6 or a space margin between the lower latch 23 and the teeth 7 of the control element drive shaft 6 is given as 1/32 inch.
Therefore, when the upper motor assembly 10 operates, the control element drive shaft 6 ascends or descends by ( 7/16- 1/32) of an inch; when the lower motor assembly 20 operates, the control element drive shaft 6 ascends or descends by (⅜- 1/32) inch.
As a result, the distance moved in the first step is different from the distance moved in the second step, and one pitch of moving the control element drive shaft 6 is completed with each sequential operation of the upper motor assembly 10 and the lower motor assembly 20.
A final one pitch completed by the operation of the upper motor assembly 10 and the lower motor assembly 20 is calculated using the following formula:Final one pitch= 7/16 of an inch− 1/32 inch+⅜ of an inch− 1/32 inch= 24/32 inch=¾ inch.
As explained herein above, the operation type of a conventional control element drive mechanism has a position control resolution of ¾ inch. However, such a conventional method is not precise enough to be used in a small reactor; therefore, there has been a demand for a control element drive mechanism with precise position control capacity.